wrap all API calls with error catching, so all API return Json-based errors
update video_download
Merge branch 'develop' of github.com:learningequality/ka-lite into safe_api_errors

Conflicts:
	kalite/securesync/api_views.py
updating formatting on video_download
making strings more i18n-friendly, being careful to only display a single error of a single type.
removing debug-only text.
Merge branch 'release-0.10' of github.com:learningequality/ka-lite into safe_api_errors

Conflicts:
	kalite/templates/video_download.html
Merge pull request #439 from bcipolli/safe_api_errors

Safe api errors
Resolved merge conflicts, removed debug-only assignment, opened issue #461